Ta có:
\(A=\left(2-1\right)\left(2+1\right)\left(2^2+1\right).....+1\)
\(=\left(2^2-1\right)\left(2^2+1\right)....\left(2^{256}+1\right)\)
\(=\left(2^4-1\right)\left(2^4+1\right)....\left(2^{256}+1\right)\)
\(......\)
\(=\left[\left(2^{256}\right)^2-1\right]+1\)
\(=2^{512}\)